As wireless technologies become more versatile and offer higher data rates, more and more mobile devices are deployed. Earlier visions of 50 billion wireless connections are fast becoming reality, while predictions of more than 500 billion connections are becoming more and more likely to be fulfilled.
In addition, mobile devices are more powerful than before. People can easily program and install new applications to their smart-phones according to their preferences. However, these capabilities and advanced features like a big screen, large memory and powerful CPU consume a lot of energy. Long battery lifetime for a mobile terminal (a “user equipment” or “UE” in the terminology of the 3rd-Generation Partnership Project, or 3GPP) is an important feature to device users—thus improved conservation of the battery's energy is required for the advancement of the UE terminal.
To satisfy the need for providing reduced power consumption and thus longer active UE time, the Long-Term Evolution (LTE) specifications developed by 3GPP include a set of functionalities to make UEs sleep when there is no need to listen to the control channel, thus extending the useful battery life. These functionalities are described in “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification (Release 9),” 3GPP TS 36.321, v. 9.4.0, November 2009.
According to said 3GPP specification, a feature called Discontinuous Reception (DRX) allows the UE to monitor the Physical Downlink Control Channel (PDCCH) only during certain time intervals specified by higher layers, for example RRC signaling, instead of monitoring the PDCCH during every transmit-time-interval (TTI). Thereby, the UE can dramatically decrease the power consumption by turning off unused circuitry during the inactive intervals. The DRX settings may provide benefits both in downlink and uplink. FIG. 1 illustrates a network node 102 serving a number of mobile terminals 104, for which the DRX functionality may be used. The network node 102 and the mobile terminals 104 in FIG. 1 are associated with a cell 106, served by the network node 102.
Below, the existing DRX functionality will be described in further detail. The DRX functionality in RRC_CONNECTED state specifies two DRX cycles for each UE, namely: the long DRX cycle and the short DRX cycle. The long DRX cycle is typically used. The short DRX cycle is an optional feature, and is configured for periods during which increased connectivity between the UE and the wireless network is needed. Within each DRX cycle, there is an active period and an inactive period. Within each DRX cycle, a UE is required to only listen to control channels for a number of consecutive PDCCH sub-frames specified by a timer called “On Duration Timer”, and can thus turn off radio-frequency (RF) circuits when this timer expires (and the inactive period begins), until the cycle begins again. When more frequent data activity is detected in downlink or uplink, the radio base station (RBS) (“evolved NodeB” or “eNB” in LTE terminology) can trigger the short DRX cycle for the UE. The transitions between long DRX cycle and short DRX cycle are trigged directly by the RBS, or driven by a predefined timer. FIG. 2 illustrates the representation of a DRX cycle comprising an active 202 and an inactive period 204. For each cycle, the terminal turns on RF only for the active “on” duration period 202, also indicated as “UE awake” in FIG. 2.
The settings for the DRX cycle can depend on the prior knowledge of traffic type. For frequent traffic types, like AMR VoIP, 20 ms of short DRX cycle fits to the VoIP frame interval. However, for traffic with a high sporadic behavior, a long DRX cycle is preferred during the periods when the traffic has low activity.
Besides the On Duration timer and the short and long cycle timers, there are also a number of other timers which may configure the active time periods for a UE:
Inactivity Timer specifies the periods during which a UE shall monitor PDCCH after successfully decoding a PDCCH assignment indicating an UL or DL data transfer for this UE.
Retransmission Timer (both UL and DL), will be triggered when a HARQ retransmission is expected.
HARQ RTT Timer (Hybrid Automatic Repeat reQuest; Round-Trip delay Time), specifies the minimum amount of subframe(s) before a DL HARQ retransmission is expected by the UE.
Timer for SR pending (Scheduling Request), if there is SR pending, UE has to listen to PDCCH until the pending SR is cancelled. When an SR is triggered, it shall be considered as pending until it is cancelled. Pending SR(s) shall be cancelled when a MAC PDU including BSR is assembled or the allocated grants can accommodate all pending data available for transmission.
As a summary, according to the specification 3GPP TS 36.321, when a DRX cycle is configured, the (UE) Active Time includes the time while on DurationTimer or drx-Inactivity Timer or drx-Retransmission Timer is running; or a Scheduling Request is sent on PUCCH and is pending; or an uplink grant for a pending HARQ retransmission can occur and there is data in the corresponding HARQ buffer; or a PDCCH indicating a new transmission addressed to the C-RNTI (Cell Radio Network Temporary Identifier) of the UE has not been received after successful reception of a Random Access Response for the preamble not selected by the UE.
When DRX is configured, the UE shall for each subframe:
if a HARQ RTT Timer expires in this subframe and the data in the soft buffer of the corresponding HARQ process was not successfully decoded:
                start the drx-Retransmission Timer for the corresponding HARQ process.if a DRX Command MAC control element is received:        stop on DurationTimer;        stop drx-InactivityTimer.if drx-InactivityTimer expires or a DRX Command MAC control element is received in this subframe:        if the Short DRX cycle is configured:                    start or restart drxShortCycleTimer;            use the Short DRX Cycle.                        else:                    use the Long DRX cycle.                        if drxShortCycleTimer expires in this subframe:                    use the Long DRX cycle.If the Short DRX Cycle is used and [(SFN*10)+subframe number] modulo (shortDRX-Cycle)=(drxStartOffset) modulo (shortDRX-Cycle); orif the Long DRX Cycle is used and [(SFN*10)+subframe number] modulo (longDRX-Cycle)=drxStartOffset:                        start on DurationTimer.during the Active Time, for a PDCCH-subframe, if the subframe is not required for uplink transmission for half-duplex FDD UE operation and if the subframe is not part of a configured measurement gap:        monitor the PDCCH;if the PDCCH indicates a DL transmission or if a DL assignment has been configured for this subframe:        start the HARQ RTT Timer for the corresponding HARQ process;        stop the drx-Retransmission Timer for the corresponding HARQ process.if the PDCCH indicates a new transmission (DL or UL):        start or restart drx-InactivityTimer.when not in Active Time, type-O-triggered SRS [2] shall not be reported.if CQI masking (cqi-Mask) is setup by upper layers:when on DurationTimer is not running, CQI/PMI/RI/PTI on PUCCH shall not be reported.else:        when not in Active Time, CQI/PMI/RI/PTI on PUCCH shall not be reported        